1. Field of the Invention
The present invention relates to an amplifier circuit of a transmitter, which is suitable for use in, for example, a CDMA (Code Division Multiple Access) mode portable telephone.
2. Description of the Related Art
In a CDMA mode portable telephone, variable gain amplifying circuits (hereinafter called "variable amplifying circuits") capable of varying a gain of 80 dB or above are normally provided at radio-frequency amplifying circuits of a transmitter unit and a receiver unit, respectively, in order to maintain communications at its movement. FIG. 3 shows a radio frequency stage of a portable telephone having general CDMA and FM dual modes. A description will first be made of a configuration of a transmission (TX) system. An IF (intermediate frequency) transmit signal modulated by a modem 101 is QPSK-modulated by a QPSK modulating circuit 102. Next, the modulated signal is amplified by a transmission-side variable amplifying circuit (TX-AMP) 103, which in turn is mixed into a local oscillating signal produced from a local oscillator (OSC) 121 by a mixer (MIX) 104, where the mixed signal is converted into an RF (radio frequency) transmit signal. The RF transmit signal is transmitted through a band-pass filter 105, a power amplifier (PA) 106, a duplexer 107 and an antenna 108.
A description will next be made of a configuration of a reception (RX) system. An RF receive signal received by the antenna 108 is applied to a mixer (MIX) 111 through the duplexer 107, a low noise amplifier (LNA) 109 and a band-pass filter 110, where the signal is mixed into a local oscillating signal generated from the local oscillator (OSC) 121 so as to be converted into an IF receive signal. The IF receive signal is applied to a CDMA band-pass filter 112 and an FM band-pass filter 113, where one output signal is selected according to a set mode and amplified by a reception-side variable amplifying circuit (RX-AMP) 114. Next, the amplified signal is demodulated by a QPSK demodulating circuit 115, followed by application to the modem 101.
The strength of the received signal, which has been detected by a receive-signal strength indicating circuit (RSSI) 116 provided within the modem 101 is compared with strength reference data by a comparator 117. The difference in strength therebetween is applied to a receive-side AGC voltage correction circuit 118 and a transmitting output correction circuit 119. The AGC voltage correction circuit 118 outputs an AGC voltage so that the difference produced from the comparator 117 becomes "0," i.e., the output of the RSSI 116 coincides with the strength reference data, thereby controlling the gain of the variable amplifying circuit (RX-AMP) 114 on the receiving side.
The difference produced from the comparator 117 and transmitting output correction data determined according to circuit conditions between a portable telephone and a base station are applied to the transmitting output correction circuit 119 on the transmitting side. An AGC voltage correction circuit 120 on the transmitting side outputs an AGC voltage so that a modulated signal is inversely proportional to the level of the received signal, and according to the transmitting output correction data, thereby controlling the gain of the variable amplifying circuit (TX-AMP) 103 on the transmitting side. In this case, excellent linearity is required between the AGC voltage and the gain over a dynamic range of 80 dB or above in order to allow the variable amplifying circuits 103 and 114 on the transmitting and receiving sides to operate in interlock with each other.
In regard to this type of amplifier circuit for the transmitter, a method of constructing both an IF amplifying circuit and an RF amplifying circuit by variable amplifying circuits and controlling the individual variable amplifying circuits with different AGC voltages, thereby realizing a dynamic range of 80 dB or above has heretofore been proposed as disclosed in Japanese Patent Application Laid-Open No. Hei 6-508012 (1994), for example.
FIG. 4 shows such a circuit. I and Q signals used as transmit signals are modulated by a QPSK modulating circuit 1, where it is converted into an IF (intermediate frequency) signal. The IF signal is applied to IF-stage variable amplifying circuits 2 and 3 provided in two stages, where it is amplified based on an AGC voltage VAGC1 used commonly to the IF stages. The IF signal is mixed into a local oscillating signal produced from a local oscillator 9 by a mixer 5, where it is converted to an RF (radio frequency) signal. The RF signal is supplied to an RF-stage variable amplifying circuit 4 where it is amplified based on an AGC voltage VAGC2 used for the RF stage. Further, the RF signal is outputted to an antenna through an amplifier 6, a band-pass filter 7 and a power amplifier 8.
However, the method of controlling the IF-stage variable amplifying circuits 2 and 3 and the RF-stage variable amplifying circuit 4 by the AGC voltages VAGC1 and VAGC2 different from each other in the above-described manner has a problem in that the control circuit becomes complex in configuration. The CDMA mode portable telephone wherein the amplification degree of the transmitter circuit needs to be controlled according to the level (RSSI) of the received signal as shown in FIG. 3, encounters difficulties in obtaining the AGC voltages VAGC1 and VAGC2 different in characteristic from each other from one receiver circuit.